Carrying out unit testing in python on a method that implements ImageDraw

I am currently experimenting with the pytest module to create unit tests for a project I'm working on. I'm trying to test the 'add_point' method which draws an ellipse based on a set of pixels. What I want to do is inspect 'draw' to ensure that the ellipse has been created successfully. Unfortunately I don't know how to go about this, so any help will be appreciated. Here's my code so far:

(A) TheSlicePreviewMaker.py

import os, Image, ImageDraw, ImageFont
from json_importer import json_importer

class SlicePreviewer(object):
    def __init__(self):
        self.screen_size = (470, 470)
        self.background_colour = (86,0,255)
        self.platform_fill_colour = (100, 100, 100)
        self.platform_outline_colour = (0, 0, 0)
        self.platform_window = (0,0,469,469)
        self.point_colour = (0,0,255)
        self.config_object = json_importer("ConfigFile.txt")
        self.image = None

    def initialise_image(self):
        self.image = Image.new('RGB',self.screen_size,self.background_colour)
        draw = ImageDraw.Draw(self.image)
        draw.rectangle(self.platform_window,outline=self.platform_outline_colour,fill=self.platform_fill_colour)
        del draw

    def add_point(self, px, py):
        x1 = px - 1
        y1 = py - 1
        x2 = px + 1
        y2 = py + 1
        draw = ImageDraw.Draw(self.image)
        draw.ellipse((x1,y1,x2,y2),outline=self.point_colour,fill=self.point_colour)
        return draw #del draw

    def save_image(self, file_name):
        self.image.save(file_name, "BMP")

(B) test_TheSlicePreviewMaker.py

from TheSlicePreviewMaker import SlicePreviewer

slice_preview = SlicePreviewer()
class TestSlicePreviewer:
    def test_init(self):
        '''check that the config file object has been created on init'''
        assert slice_preview.config_object != None

    def test_initialise_image(self):
        '''verify if the image has been successfully initialised'''
        assert slice_preview.image.mode == 'RGB'

    def test_add_point(self):
        '''has the point been drawn successfully?'''
        draw = slice_preview.add_point(196,273)
        assert something

import pytest
if __name__ == '__main__':
    pytest.main("--capture=sys -v")

SN: I've run TheSlicePreviewMaker.py separately to check the bitmap file it produces, so I know that the code works. What I want to achieve is unit test this so that each time I don't have to go check the bitmap.